Here's how it could be done (I'm no specialist in types of servos/motors available).

As you can see in my kick drawing, the rod would go through the housing at the top (of course) but also through the bottom of the machine.
You'd gain some room by cheating since the rod will never be seen going under the prop.
Also, to reduce the length of the push rod, you'll have to place it in a toward fashion (close to the attachment that's securing the bellows at the right hand side of the plan) rather than putting toward the end (left hand side).

You would still have room for the motor lifting the arm/eye combo (I hope it helps).

Surely the mechanically simplest way to implement the bellows mechanism must be just a rotating cam, something like the image below?

That way, you can just let the motor keep on going steadily in one direction, and the bellows will obligingly move up and down; the speed of the motion can easily be controlled through the speed of the DC motor.

I don't think it actually exists as such, but it's easy enough to make. Any circular disc of suitable diameter to act as a cam, and a fairly simple, off-the-shelf DC motor assembly (you have a ton of options here), and off you go.

That same place actually offers various fun options with linear servos and various other toys, but getting the same even movement you get with just a cam, a motor, a switch, and a battery using pretty much any other type of mechanism is a (a) a lot trickier and (b) more expensive. Reason why internal combustion engines still, 130-plus years later, use camshafts..._________________26354

That is pretty much the solution I've been playing with in my research for this beast.... couldn't you you just fix a rod to the perimeter of a rotation disc to push the bellows?_________________Monsieur, azonnal kövessen engem bitte

That is the design I'm going to experiment with on mine... but I really like the idea with the cam!! Will the bellows own weight be enough to lower them after each turn?_________________Monsieur, azonnal kövessen engem bitte

I like the idea of the off center disc (reminds me of the vibration motors in some old game pads - motor with off center weight attached). I had thought of the disc with a rod attached like in combustion engines. Biggest issue seems to be how far the bellows need to move and if there is room to contain the mechanism (I haven't done the measurements yet).
Another possible mechanism is something like an auto scissor jack:
[img]http://6505at3mrb.weebly.com/uploads/1/9/7/2/19720361/6333149_orig.jpg?1369556094[/img]

In reply to Joberg, the stepper motor for the arm raise is 42mm high.

8th_Passenger's work looks amazing.

I got side tracked into putting together a mock up of the screen side (I think of it as the examiners side) for pretty much the same reasons that 8th_Passenger build up models of the smaller screens - I just needed to see the proportions, confirm measurements. etc. Mine is a bit cruder, just card stock and foam:
I'm also trying to build up a 3D CAD model like 8th_Passenger (his model seems quite a bit more developed ):
You can access my design files at https://cad.onshape.com/documents/cb7c199fecd1c9d080129028/w/ced4bad1bf548e19fb4c3765/e/a74888ee4c1af21c5230ee05 You may need to register for a free account but the files are publicly accessible._________________If only you could see the things I've seen with your eyes.

Trying to re work measurements. What is the current consensus for the size of white buttons on the lower right (examiner side) on the VK (1.2, 1.5, 1.7)?

Working out bugs in my code for the arm raise, extension and rotation (adding magnetic end stops to trigger each stage).

Have some working prototypes of the LED bar graphs, both of them (above main screen and below smaller screens). A summary video of progress on this: https://youtu.be/tRoTafkjqy8

Dimensions for my current prototype for the LED bar graph above the main screen are: individual LED's 2x5mm, length of all 12 LED's together (including spacing) is 73.5mm. This is without the thin black and thicker white outline._________________If only you could see the things I've seen with your eyes.

Arrgh!!! Looking at 8th_Passengers thread at http://www.propsummit.com/viewtopic.php?t=5242&postdays=0&postorder=asc&start=20 and at my calculations using white buttons at 1.5cm (instead of 1.7cm - which I used before) I get a 415cm wide VK which makes the two LED bar graphs 32.1mm wide and 73.4mm wide. So I need to either space the LED's for the longer bar graph closer, or use different LED's and for the smaller bar graph I need a module just a bit larger (mine is 25.4mm wide). Well at least the circuit layout and code seems okay._________________If only you could see the things I've seen with your eyes.

Think I have a solution for the big LED bar graph, after looking for different sizes of rectangular LED's. Using 12 2x5 LED's gives a minimum of 60mm wide, adding 2.5mm space on each end - up to 65mm, add 1mm spacing between each LED is another 11mm so up to 76mm (just 2.6mm to long). Most of the structure of LED's is just housing, the sides can be sanded down to reduce there size (as long as you don't sand of the leads or cut into the lens or the tiny semiconductor chip). I've successfully done this with standard 5mm round LED's, now it's just a challenge to figure out the size and spacing to aim for!

Still need a substitute for the smaller bar graph - likely need to fabricate also._________________If only you could see the things I've seen with your eyes.